This is an improvement to the apparatus for producing glass filaments disclosed in U.S. Ser. No. 317,854, filed Dec. 22, 1972, by Walter W. Harris, now abandoned. This specification adopts that portion of the disclosure of the earlier application necessary to the teaching of the present improvement.
Glass filaments are produced by attenuating continuous threads of molten glass from a plurality of small tips or orifices located in a bottom wall of a heated vessel called a bushing. The molten glass forms globules at discharge ends of the tips. Due to the attenuation of the glass which is being drawn from the globules into filaments, these glass globules are shaped like inverted cones. With this type of forming process, the original viscosity of the glass making up each globule and the viscosity change of the glass in each globule from the base to the apex of the cone affects the diameter of the filament formed and the production rate of the filament forming process. The more uniform the original viscosity is for the globules and the more uniform the change in viscosity is among the globules, the more uniform the filament diameters and the better the production rate.
Since the initial viscosity of the glass and the change in the viscosity of the glass from the base to the apex of a cone is directly related to the temperature of the glass, the quality of the filaments in terms of uniform diameter and the production rate of the filament forming process depends to a large extent on the temperatures of the tips and the molten glass globules formed at the tips. The more uniform the temperatures and temperature changes are, the more uniform the diameters of the various filaments are and the better the production rate is for a particular diameter filament.
If the inverted glass cone at the tip is subjected to an excessive temperature, the viscosity of the cone is reduced to such an extent that the molten glass can no longer support the continuous filament depending therefrom and the filament breaks off. This condition is referred to as streaking or burn-off. The deviation between the burn-off temperature of the tip and the operating temperature required for maximum productivity is a judgment factor generally established by trial and error while operating the bushing. There must be a satisfactory margin between the burn-off temperature and the actual operating temperature of the bushing to compensate for viscosity changes caused by variations in the homogeneity of the glass, the uncontrolled variations of tip temperature due to minor inaccuracies in the power control response, tip configuration, or other factors which can affect the temperature of the bushing.
In practice, bushings have multiple tips, e.g., 400, 800, 1,600, 2,400. Therefore, the maximum melt rate or productivity which can be obtained depends upon the hottest tip or group of tips characteristic of the particular bushing design for the temperature of these tips must be regulated so as to be below the burn-off temperature. To do this, other tips which are not as hot will have temperatures farther away from the optimum temperature with the amount of temperature variation between the tips determining how far from the optimum temperature the other tips are operating. For the bushing to perform satisfactorily, the temperature of the coldest tips at the outside rows must exceed a certain value or cold beads will form thus interrupting the continuous drawing operation. In other words, the tip temperature among all tips must fall within a specific range so that the glass viscosity will remain suitable for continuous attenuation. If the temperature variation is too great, one will experience burn-off at the hotter tips and/or beading at the coldest tips. In addition, even if neither of these conditions occurs if the temperature variation is large enough, the bushing will not operate at a very high efficiency. While some of the tips may be operating at a satisfactory margin with respect to the burn-off temperature thereby optimizing the production of these tips other tips may be operating at a temperature variation which, although not sufficient to cause bead-out will cause the flow of glass from those orifices to be considerably less than the desired flow rate. Consequently, it is desirable to maintain the various tips at a uniform temperature so that the tips can be operated as close as possible to the burn-off temperature.
The temperature of a cone can drop about 1,350.degree. F. in a fraction of an inch between the base of the cone at the tip and the apex of the cone. The outside tip rows are exposed to the relatively cool ambient atmosphere. In addition, the rapidly moving filaments (for example, 800 to 1,600 filaments moving at 10,000 feet per minute) act as an air pump drawing ambient air onto the surface of the outside tips to thereby cool these tips. In contrast, the inner tips receive radiant heat from adjacent tips and the thermal effect of the air currents produced by the filaments drawn from these tips are quite different from those caused by the air currents generated by the filaments of the outer rows which drawn in the ambient air. Consequently, the temperature drop in the glass globules of the inner and outer tip rows can vary as much as 100.degree. F.
Another factor which makes it difficult to maintain a uniform temperature among the many glass globules is the poor temperature balance of the molten glass at various levels in the bushings itself. This is particularly true at levels near the bottom of the bushing. In other words, the temperature isotherms in the bushing are not planar. A majority of the heat in the molten glass globules comes from the molten glass in the bushing. Consequently, with nonuniform glass temperatures in the bushing above the tips, the temperatures of the glass in the tips will not be uniform In fact, one of the prime objectives of bushing development is to obtain planar temperature isotherms in the lower levels of the bushings.
From the above, it is clear that one of the main objectives in the design of a bushing for either the direct melt or the marble melt process is to obtain an even temperature profile across the tip section of the bushing. In other words, the temperature must be as uniform as possible among the various tips. This is required to achieve a maximum productivity or operating efficiency. It is also required to obtain the best quality fiber since the uniformity of the fiber diameter, which is a prime requisite for many end uses, depends upon obtaining the same glass flow from each tip which, in turn, depends to a large extent upon maintaining a uniform temperature among the tips.
To improve the efficiency of glass filament production, it has been necessary to provide external cooling devices positioned close to the bushing tips to prevent the inner tips from overheating. This is generally done by the use of water cooled tubes or solid heat conduction fins connected to a water header located outside of the bushing tip area and generally on one side of the bushing. The tubes or fins are generally located between pairs of tip rows with each pair of tip rows being spaced from adjacent pairs of tip rows.
The service demands on the water cooled tubes and the heat conduction fins are extreme. They must be able to withstand the high temperatures associated with glass fiber production in a corrosive environment. Further, such heat exchange devices must possess requisite mechanical strength, but not be so rigid as to be unbendable or unformable to match the surface contour of the bushing tip plate over its service life.
In practice these tubes and fins have been formed of an alloy constituted predominantly of platinum-group metals, e.g., an alloy of 80% platinum--20% rhodium by weight. Only precious metal alloys of this class were known to possess the physical properties necessary to meet the service demands. However, given the scarcity and expense of these metals their use defined a problem which the present invention addresses. Other non-precious metals such as copper, nickel and Inconnel have been tried, but all have failed due to the temperature and corrosive conditions in the environment of the cooling tubes and heat conduction fins.